Synthesis of metabolically blocked paclitaxel analogues

Article abstract of DOI:10.1016/S0960-894X(01)00066-X

The stereospecific syntheses of the metabolically blocked 6-α-F, Cl, Br paclitaxel, and 6-α-F-10-acetyldocetaxel are described and in vitro and in vivo activity is presented.

Full text of DOI:10.1016/S0960-894X(01)00066-X

Bioorganic & Medicinal ChemistryLetters 11 (2001) 809±810
Synthesis of Metabolically Blocked Paclitaxel Analogues
Mark D. Wittman,a,* Thomas J. Altstadt,a Craig Fairchild,b Stephen Hansel,c
b
KathyJohnston, John F. Kadow,a Byron H. Long,b William C. Rose,b
c
Dolatrai M. Vyas,a Mu-Jen Wua and MaryE. Zoeckler
aDiscovery Chemistry, Bristol-Myers Squibb Pharmaceutical Research Institute, PO Box 5100, Wallingford, CT 06492-7660, USA
bOncology Drug Discovery, Bristol-Myers Squibb Pharmaceutical Research Institute, PO Box 4000, Princeton, NJ 08543-4000, USA
cDepartment of Metabolism and Pharmacokinetics, Bristol-Myers Squibb Pharmaceutical Research Institute, PO Box 5100, Wallingford,
CT 06492-7660, USA
Received 1 November 2000; accepted 25 January2001
AbstractÐThe stereospeci®c syntheses of the metabolically blocked 6-a-F, Cl, Br paclitaxel, and 6-a-F-10-acetyldocetaxel are
described and in vitro and in vivo activityis presented. # 2001 Elsevier Science Ltd. All rights reserved.
Several studies have elucidated the metabolic fate of
paclitaxel in humans, rats, and mice. The major
human metabolite of paclitaxel has been identi®ed as
6-a-hydroxypaclitaxel and is 30-fold less active than
paclitaxel itself.1,2 Studies in humans have revealed a
signi®cant portion of the dose of paclitaxel is excre-
ted through the bile as 6-a-hydroxypaclitaxel.3 Thus,
6-hydroxylation of paclitaxel represents a route for the
detoxi®cation and elimination of paclitaxel in humans.
Blockade of this metabolic pathwayshould therefore
provide taxanes with improved therapeutic ecacyand
reduced clearance. Metabolic stabilization could also
reduce the toxicityassociated with taxane treatment by
reducing the dose needed to achieve antitumor ecacy.
For this reason, we became interested in preparing taxane
analogues that maintained antitumor ecacybut would
be less prone to metabolic deactivation via 6-a-hydroxy-
lation. We felt that introduction of an a-¯uorine would
Scheme 1. Reagents and conditions: (a) 4-BzO-TEMPO (2%), KBr, Chlorox1; (b) silica gel, CH2Cl2; (c) Na(OAc)3BH, HOAc, CH3CN (75%
overall yield for a±c); (d) SOCl2, Et3N; NaIO4, RuCl3, CCl4/CH3CN/H2O (77%); (e) Bu4NX (F, Cl, Br), THF (76%); (f) 1 N HCl, acetonitrile
(85%).
*Corresponding author. Tel.:+1-203-677-6318; fax: +1-203-677-7702; e-mail: mark.wittman@bms.com
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00066-X

810
M. D. Wittman et al. / Bioorg. Med. Chem. Lett. 11 (2001) 809±810
Table 1. Biological activityof C-6 a halogenated taxanes
gues. The potential advantages of these analogues in the
clinic could not be adequatelytested in murine model
systems since both 6-a hydroxylation and 30-p-phenyl
hydroxylation pathways are observed in mice10 as
opposed to humans in which 6-hydroxylation pre-
dominates. To address the potential for improved
metabolic stabilityin humans the analogues were eval-
uated in a human liver S9 fraction capable of hydro-
xylating paclitaxel.11,4 There was no detectable metabolite
formation from the halogenated taxanes 11a±c in com-
parison with paclitaxel which generated detectable levels
of the 6-a hydroxylated metabolite. These results show
that 6-a halogenation does block to some degree the
major metabolic pathwayfor paclitaxel.
Compound
Tubulina IC50 HCT-116 (nM)
M109b
Paclitaxel
11a
11b
11c
12
10-Acetyldocetaxel
1.0
1.0
0.8
1.1
0.6
0.7
2±4
1.4
1.2
2.6
0.2
1.8
197±215 (40, 60)
c
203 (200)
214 (200)
269 (100)
177 (32)
aTubulin assaymeasures the initial rate of polmy erization and is
expressed as a ratio to that obtained for paclitaxel. Ratios less than 1
re¯ect analogues that show a more rapid polymerization rate than
paclitaxel (see ref 6).
b%T/C analogue at the maximum tolerated dose (dose mg/kg). %T/C
values of greater than 125 % are considered active.
cNot determined in this model system.
References and Notes
not signi®cantlyalter the biological activityof paclitaxel
but would e€ectivelyblock 6- a hydroxylation. With this
goal in mind, we turned to chemistrywe had developed
for the synthesis of 6-a-hydroxy paclitaxel.4
1. Harris, J. W.; Katki, A.; Anderson, L. W.; Chmurny, G. N.;
Paukstelis, J. V.; Collins, J. M. J. Med. Chem. 1994, 37, 706.
2. Sparreboom, A.; Huizing, M. T.; Boesen, J. J. B.; Nooijen,
W. J.; van Tellingen, O.; Beijnen, J. H. Cancer Chemother.
Pharmacol. 1995, 36, 299.
3. Walle, T.; Walle, U. K.; Kumar, G. N.; Bhalla, K. N. Drug
Metab. Dispos. 1995, 23, 506.
4. Wittman, M. D.; Kadow, J. K.; Vyas, D. M. Tetrahedron
Lett. 2000, 41, 4729.
5. Gao, Y.; Sharpless, K. B. J. Am. Chem. Soc. 1988, 110,
7538.
6. Schi€, P. B.; Fant, J.; Horwitz, S. B. Nature 1979, 277, 665 .
An E0.01was determined for each analogue and expressed as a
ratio relative to paclitaxel.
7. Scudiero, D. A.; Shoemaker, R. H.; Paull, K. D.; Monks,
A.; Tieney, S.; Nofziger, T. H.; Currens, M. J.; Seni€, D.;
Boyd, M. R. Cancer Res. 1988, 48, 4827. The IC50 for each
compound was determined against HCT-116, a human colon
tumor cell line sensitive to paclitaxel.
8. %T/C values of greater than 125 are consider active: Rose,
W. C. Cancer Treat. Rep. 1981, 65, 299.
9. Rose, W. C. Anti-Cancer Drugs 1992, 3, 311.
Previously, we demonstrated a convenient procedure for
conversion of the readilyavailable C-6,7- a-diol to the
corresponding b-diol.4 The b-diol, 7, served as a useful
intermediate for introducing functionalityat the 6- a-
position via the cyclic sulfate since nucleophiles regio-
selectivelyadd to the 6 position. The regioselectivity
observed for the addition of nucleophiles to the cyclic
sulfate results from the steric compression between the
7 position and the B ring acetate bearing methine. The
cyclic sulfate obtained from diol 7 was treated with
¯uoride,5 chloride, and bromide ion to provide 6-a halo
analogues 9a±c. Deprotection under acidic conditions
a€orded the targeted taxane analogues 11a±c. This
sequence was repeated with the docetaxel side chain to
provide 6-a-¯uoro-10-acetyldocetaxel, 12 (Scheme 1).
Analogues 11a±c and 12 were evaluated for inhibition
of tubulin polymerization6 and cytotoxicity against the
human colon tumor cell line, HCT-116.7 Analogues
11a±c and 12 had in vitro activityindistinguishable
from paclitaxel or C-10-acetyldocetaxel. Analogues
11b,c and 12 were evaluated in vivo using the M109
Madison murine lung carcinoma tumor model.8 The
in vivo activityof 11b,c was equivalent to that observed
for a reference dose of paclitaxel9 (known to be quite
e€ective but not necessarilyoptimized). The 6- a-¯uoro-
10-acetyldocetaxel, 12, did produce an increase in life-
span greater than that observed for 10-acetyldocetaxel
and paclitaxel in this model (Table 1). These results
suggest that halogenation of the 6-a position does not
alter the in vitro or in vivo ecacyof paclitaxel analo-
10. Sparreboom, A.; van Tellingen, O.; Nooijen, W. J.; Beij-
nen, J. H.; Jos, H. Anti-Cancer Drugs 1996, 7, 78.
11. A pooled lot of P450 characterized human liver S-9 frac-
tions were obtained from The International Institute for the
Advancement of Medicine. All incubations were performed in
duplicate at concn of 12.5 mg/mL (solubilized in 1.25% ethanol
®nal volume) in S-9 preparations containing approximately
5 mg microsomal protein/mL, phosphate bu€er (0.1 M pH 7.4)
and an NADPH regenerating system. Incubations were per-
formed at 37 ꢀC for 120 min in a shaking water bath. Sample
aliquots were analyzed by HPLC/UV after acetonitrile depro-
teinization. Paclitaxel was used as a control and generated
detectable amounts of 6-a-hydroxy paclitaxel. Due to the
limited extent of paclitaxel metabolism in this system it is
dicult to make de®nitive conclusions about the extent to
which analogues 11a±c block this metabolic pathway.